The invention relates generally to an electrostatically effective air filter material that comprises a blend of (I) polyolefin fibers and (II) polyacrylonitrile fibers. In filter material of this type, each of the constituent fibers is free of lubricants and antistatic agents, and any burnishing substances. The present invention also concerns the corresponding process by which such material is manufactured, and the manner in which the air filter material is utilized.
An air filter material of this kind is described in EP-B1 0 246 811 and its corresponding U.S. Pat. No. 4,798,850 (the contents of which are incorporated herein by reference). This patent discloses a procedure wherein polyolefin fibers that are free of lubricants and antistatic agents are mixed with fibers of an addition polymer that also are free of lubricants and antistatic agents. The addition polymer comprises one or more halogen-substituted polyolefins as well as, for example, acrylonitrile units. This goal of that process is the production of an electrostatically effective air filter material having low material costs and high filtration performance, which presents little resistance to the flow of air, and is not flammable. Such an air filter material is said to be suitable for use in dust-protection helmets and masks, among other uses.
An essential feature of this process is that the fibers must be cleansed before being mixed together, so that they do not have a coating of lubricants or antistatic agents. This cleaning is performed, for example, by rinsing the fibers with a nonionic detergent, with alkali, or with solvent, and then rinsing them off in a subsequent rinse procedure.
The resulting air filter material possesses the desired technical attributes for such a filter material, but has the drawback of limited packageability. This material can be manufactured in a geometrically stable manner, e.g., into anatomically shaped breathing masks, only if it is provided with an additional stiffening reinforcement ply.
EP-B1 0 246 811 teaches that the nonwoven fabric be "lightly" needled at 25 punctures/cm. To manufacture stiffer or denser nonwoven mats, it merely suggests more intensive needling. In many cases, however, this action does not lead to the desired stiffness that is achieved with reinforcement plies. The latter, however, always requires the use of additional material, and also considerably increases the flow resistance.
This invention is directed to the problem of providing an air filter material of this general type and efficacy, but without the drawbacks of the known approaches. In particular, this invention permits the formation of geometrically stable air filters into such shapes as may be required for use in breathing masks or other air-delivering devices without the need for an additional reinforcement ply.